Learn about Congealing temperature and the procedure for congealing temperature apparatus
Definition
The temperature at which a substance passes from the liquid to the state upon cooling is a useful index to purity if heat is liberated when the solidification takes place, provided that any impurities present dissolve in the liquid only, and not in the solid. Pure substances have a well-defined freezing point, but mixtures generally freeze over a range of temperatures. For many mixtures, the congealing temperature, as determined by strict adherence to the following empirical, is a useful index of purity. The method for determining congealing temperatures set forth here is applicable to substances that melt between -200 and 150o, the range of the thermometer used in the bath. The congealing temperature is the maximum point (or lacking a maximum, the point of inflection) in the temperature- time curve.
Congealing Temperature Apparatus
The container for the substance is a 25 x 100-mm test tube. This is provided with a suitable, short range thermometer suspended in the center, and a wire stirrer, about 30cm long, bent at its lower end into horizontal loop around the thermometer. Use a thermometer having a range nit exceeding 300C, graduated in 0.10 divisions, and calibrated for, but not used at, 76-mm immersion. A suitable series of thermometers, covering a range from -200 to + 150o, is available as the ASTM E1 series 89C through 96C. Other temperature- measuring devices may be used if they are validated for this procedure. Dimensions should be within ±20%.
The specimen container is supported, by means of a cork, and a suitable water-tight cylinder about 50mm in internal diameter and 11cm in length. The cylinder, in turn, is supported in a suitable bath sufficient to provide not less than a 37-mm layer surrounding the sides and bottom of the cylinder. The outside bath is provided with a suitable thermometer.
Procedure
Melt the substance, if a solid, at a temperature not exceeding 20oC above its expected congealing point, and pour it into the test tube to a height of 50 to 57mm. Assemble the apparatus with the bulb of the test thermometer immersed halfway between the top and bottom of the specimen in the test tube. Fill the bath to about 12mm from the top of the tube with suitable fluid at a temperature 4o to 5 o below the expected congealing point.
When the test specimen has cooled to about its expected congealing point, adjust the bath to a temperature 7 o to 8 o below the expected congealing point. Stir the specimen continuously during the remainder of the test by moving the loop up and down between the top and bottom of the specimen, at a regular rate of 20 complete cycles per minute.
Congelation frequently may be induced by rubbing the inner walls of the test tube with the thermometer or by introducing a small fragment of the previously congealed substance. Pronounced super cooling may cause deviation from the normal pattern of temperature changes. If the latter occurs, repeat the test, introducing small particles of the material under test in solid form at 1 o intervals as the temperature approaches the expected congealing point.
Record the reading of the test tube thermometer every 30 seconds. Continue stirring only so long as the temperature is gradually falling, stopping when the temperature becomes constant or starts to rise slightly. Continue recording the temperature in the test tube every 30 seconds for at least 3 minutes after the temperature again begins to fall after remaining constant.
The average of not less than four consecutive readings that lie within a range of 0.2 o constitutes the congealing temperature. These readings lie about a point of inflection or maximum, in the temperature-time curve, that occurs after the temperature becomes constant or starts to rise and before it again begins to fall. The average to the nearest 0.1 o is the congealing temperature.
Solidification, congealing temperature, temperature- time curve
